1. Introduction
Camellia sinensis, classified in the Camellia family and Camellia genus, is a perennial evergreen woody plant. Its leaves can be processed into tea, rich in beneficial compounds like tea polyphenols, amino acids, and caffeine, which exhibit anticancer, anti-inflammatory, and cardiovascular benefits [
1]. The tea tree’s fruit, known as tea seeds, yields oil containing physiologically active components such as fatty acids, vitamin E, and squalene. This oil has shown promise for improving cognitive function and reducing neurotoxicity [
2,
3,
4]. In an ovariectomized mouse model, tea seed oil, with its high levels of monounsaturated fatty acids, was found to prevent obesity and reduce fatigue compared with soybean oil and lard [
5].
China’s tea gardens span over 47 million acres, yet the self-produced edible oil is less than 30%. In recent years, numerous studies have concentrated on enhancing tea seed yields, primarily by innovating tea tree cultivation models. However, there has been limited research on the development of tea seeds themselves. Fatty acid synthesis and regulation mechanisms have predominantly been explored in economic oil crops like rapeseed and soybeans [
6,
7,
8], with woody oil plants focused on
Camellia oleifera and walnuts [
9]. A recent study revealed that the photosynthetic traits of plants significantly influence fruit quality, with metabolomics serving as a crucial tool in the study of these biological characteristics [
10]. Utilizing mass spectrometry (MS) metabolomics analysis on tea at various developmental stages, it was found that changes in the carbon pool during leaf growth are related to fatty acid synthesis [
11]. Studying the mechanism of fatty acid synthesis during the peak period of tea tree oil conversion holds significant potential for targeted cultivation and high yields in tea tree production.
Tea leaves are the primary source for producing tea beverages, while tea seeds are processed to extract tea oil. To optimize the utilization of tea seeds from the tea plant, we have chosen Wuniuzao and Jincha 2 cultivars for our study. Wuniuzao is the primary cultivar cultivated in Zhejiang province for both tea and tea seed oil production, whereas Jincha 2 is a novel cultivar bred by our institute that has better tea leaf quality, a higher yield of tea leaves, and a higher seed oil content compared with the Wuniuzao cultivar. Combining insights from photosynthetic characteristics and metabolomics research, we illuminated how the differences in photosynthetic efficiency and metabolism during the peak oil transformation period in Jincha 2 and Wuniuzao affect the biological characteristics, oil content, and fatty acid composition of mature tea seeds. We propose to lay a theoretical foundation for achieving a dual harvest of seeds and leaves, ultimately enhancing the overall benefits of tea gardens.
4. Discussion
During the peak oil transformation period, we found photosynthesis and metabolites as key factors contributing to the oil content in tea seeds. Leaves are the primary site for plant photosynthesis, serving as the basis for material accumulation. The accumulation and transport of materials are intricately linked to grain yield [
21]. Jincha 2 exhibited a high net photosynthetic rate, actual photon yield, and electron transfer rate during the peak oil conversion phase, indicating its exceptional efficiency in harnessing light energy for the synthesis of photosynthetic products and fruit development.
The KEGG enrichment analysis unveiled significant differences in metabolites during the oil conversion period, particularly in the TCA cycle and pyrimidine metabolism (
Figure 5). The TCA cycle, vital for energy generation, participates in numerous metabolic pathways such as hormone signaling and glycolysis [
22,
23]. Acetyl CoA can enter the TCA cycle to produce ATP [
24] and serve as a precursor for fatty acid synthesis [
25]. Interestingly, levels of aconitic acid, citric acid, and malic acid, key components of the TCA cycle, were significantly lower in Jincha 2 than those in Wuniuzao (
Figure 6). However, Jincha 2 displayed a significantly higher net photosynthetic rate, actual photon yield, and electron transfer rate compared with Wuniuzao. This suggests that the photosynthetic metabolites in Jincha 2 more actively yield acetyl CoA, participating in fatty acid synthesis.
The biosynthesis of pyrimidine follows a highly conserved pathway across species, involving the conversion of orotic acid into uracil [
26]. Furthermore, uracil oxidase catalyzes the transformation of uracil into malonic acid [
27,
28], which, in turn, synthesizes malonic acid coenzyme A with the assistance of methylmalonic acid coenzyme A synthase. Malonic acid coenzyme A functions as an essential metabolic intermediate of malonate [
29] and contributes to fatty acid biosynthesis as a carbon donor substrate [
30]. Our findings indicate a significant reduction in intermediate metabolites within the pyrimidine metabolism pathway in Jincha 2 compared with Wuniuzao (
Figure 6). This suggests that in Jincha 2 fruit, malonic acid coenzyme A may be directed towards the synthesis of fatty acids in tea seeds.
Furthermore, fatty acid synthase (FAS) biosynthesizes fatty acids via Claisen-like condensations of malonyl-CoA, while polyketide synthases (PKS) share a similar biosynthetic pathway with FAS, utilizing the same precursors and cofactors. PKS also plays a pivotal role in the synthesis of secondary metabolites, such as flavonoid [
31]. In our current study, we observed lower levels of secondary metabolites in the Jincha 2 cultivar compared with Wuniuzao, suggesting these synthases are more actively involved in fatty acid biosynthesis.
Plant hormones play an important role in the regulation of fruit development, orchestrating intricate interactions that govern various facets of this process [
32]. GA-1 and IAA, classified as auxins, are known to stimulate plant growth and elevate crop yields [
33]. Our study revealed higher levels of GA-1 and IAA in Jincha 2 compared with Wuniuzao. Previous studies have indicated that GA can promote leaf chlorophyll synthesis, prevent chlorophyll degradation, and increase chlorophyll content [
34,
35]. Chlorophyll, in turn, enhances crop photosynthesis, serving as the foundation of plant growth and development [
36]. Furthermore, studies have illuminated the role of four plant hormones, including IAA and GA, in upregulating genes responsible for fatty acid synthesis in green algae, with IAA having the greatest stimulating effect on the fatty acid content in
Chlorella [
37].
Additionally, we observed reduced levels of ABA, ABA-GE, and zeatin in Jincha 2 compared with Wuniuzao. ABA, which is usually released from inactive ABA-glucose ester (ABA-GE), plays important roles in seed maturation processes [
38], providing an explanation for the smaller fruit size observed in Jincha 2 compared with Wuniuzao. Zeatin, a prevalent cytokinin, typically stimulates plant growth by promoting cell division. Cytokinins act as antagonists to IAA in determining apical dominance [
39]. Du et al. (2023) [
40] indicated that fatty acid desaturase 2 (FAD2) inhibits cytokinin synthesis in peanuts using scRNA-seq, suggesting zeatin is negatively correlated with fatty acid biosynthesis. Collectively, Jincha 2 exhibited significantly higher levels of GA and IAA and lower levels of zeatin compared with Wuniuzao (
Figure 7), potentially enhancing photosynthesis and resulting in the generation of more photosynthetic products in tea fruits. Consequently, this increases oil content and plant yields in Jincha 2 compared with Wuniuzao.
5. Conclusions
In summary, during the peak period of oil conversion, the higher net photosynthetic rate and chlorophyll fluorescence in Jincha 2 promote photosynthesis, facilitating the synthesis of more organic products. This, in turn, enhances fruit growth and quality, ultimately leading to a higher yield per plant at the maturity stage in Jincha 2 than in Wuniuzao. Through metabolomics analysis, we identified 94 metabolites with significant differences. Notably, Jincha 2 exhibited higher levels of GA1 and IAA, possibly contributing to increased levels of unsaturated fatty acids and higher oil content in Jincha 2 fruit by regulating photosynthesis. Additionally, the intermediate metabolites for fatty acid synthesis were increased in Jincha 2 compared with Wuniuzao. These findings provide a solid foundation for deeper exploration of fatty acid synthesis mechanisms among tea varieties, targeted improvement of tea breeding, and the realization of a double harvest of seeds and leaves.